Observation of a JPC=1-+ Exotic Resonance in Diffractive Dissociation of 190 GeV/c π- into π-π-π+

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the pi(-)pi(-)pi(+) final state using a 190 GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420 000 events taken at values of the squared 4-momentum transfer t(l) between 0.1 and 1 GeV2/c(2). The well-known resonances a(1)(1260), a(2)(1320), and pi(2)(1670) are clearly observed. In addition, the data show a significant natural-parity exchange production of a resonance with spin-exotic quantum numbers J(PC) = 1(-+) at 1.66 GeV/c(2) decaying to rho pi. The resonant nature of this wave is evident from the mass-dependent phase differences to the J(PC) = 2(-+) and 1(++) waves. From a mass-dependent fit a resonance mass of (1660 +/- 10(-64)(+0)) MeV/c(2) and a width of (269 +/- 21(-64)(+42)) MeV/c(2) are deduced, with an intensity of (1.7 +/- 0.2)% of the total intensity.